1. Field of the Invention
The present invention relates to a method for producing a high quality silicon single crystal ingot free of point defects, and in particular, to a method for producing a high quality silicon single crystal ingot in which growth defects are controlled by controlling the temperature distribution of a melt when growing a silicon single crystal ingot by the Czochralski method.
2. Description of the Related Art
Conventional techniques have controlled the temperature distribution of a high temperature region of a single crystal ingot after crystallization so as to grow a high quality silicon single crystal ingot which may lead to increased yield of a semiconductor device. This intends to control stresses which are induced by shrinkage due to cooling after crystallization or to control the movement of point defects which occurred during solidification.
Generally, a method for producing a silicon single crystal ingot by the Czochralski method loads a polycrystalline silicon in a quartz crucible, melts the polycrystalline silicon by heat radiated from a heater into a silicon melt, and grows a silicon single crystal ingot from the surface of the silicon melt.
For growing a silicon single crystal ingot, the crucible is rotated by an axis which supports the crucible and is elevated upwardly so that a single crystal is coplanar with the surface of a melt. The silicon single crystal ingot is pulled up while being rotated in the opposite direction to the rotational direction of the crucible The grown silicon single crystal ingot goes through a series of wafer fabrication processes, for example slicing, lapping, polishing and cleaning processes, to form a silicon single crystal wafer for a semiconductor device substrate.
A prior art, for example U.S. Pat. No. 6,045,610, teaches a high quality ingot or wafer free of defects and a method for producing the same which control Δ(V/G) based on Voronkov's theory. According to Voronkov's theory, if a V/G ratio is above the critical point, a dominant point defect in a crystal is vacancy-rich, and if a V/G ratio is below the critical point, a dominant point defect in a crystal is interstitial-rich, where V is a pulling rate influencing a convection of a point defect in the silicon single crystal, and G is an instantaneous axial temperature gradient in the vicinity of an interface of a crystal melt influencing a diffusion of a point defect by forming the temperature gradient in a crystal.
If G is dominant (G is larger than V), a large temperature gradient leads to a large thermodynamic density gradient of a point defect, thus resulting in a large diffusion of point defect in a crystal direction. In this case, because mobility of interstitial defect is larger than that of vacancy defect, a dominant point defect becomes interstitial defect. On the contrary, if V is dominant (V is larger than G), the state that a vacancy concentration is large than an interstitial concentration at the crystal-melt interface maintains during the crystal growth due to convention occurred in the melt by pulling up a single crystal (because influence of G is trifle). Therefore, if a V/G ratio is above the critical point, vacancy-rich crystal ingot is generated, and if a V/G is below the critical point, interstitial-rich crystal ingot is generated.
Another prior arts, for example KR Patent Registration No. 374703, KR Patent Registration No. 411571 and U.S. Pat. No. 6,527,859, suggest an additional structure such as a heat shield for controlling an axial temperature gradient (G) of a growing silicon single crystal ingot in the radial direction of a crystal. The use of the heat shield is effective in controlling the temperature gradient of a circumferential part of a single crystal, but it has a limitation in controlling the temperature gradient of a central part of the single crystal.
Another prior art, for example KR Laid-open Patent Publication No. 2004-84728, discloses a method for producing a high quality ingot or wafer free of defects by controlling the temperature distribution in a melt using a co-rotation method that makes the rotational direction of a single crystal ingot and the rotational direction of a crucible equal. However, the co-rotation method has the drawback that the temperature distribution in a melt is deteriorated by various control factors and the control of oxygen concentration is difficult.
Another prior art has intended to control an axial temperature gradient by controlling various process parameters of a single crystal growth process. However, control of process parameters is insufficient for controlling an axial temperature gradient of a silicon single crystal ingot as desired. Further, the art has a difficulty in producing a high quality silicon single crystal ingot having a low point defect density with a high productivity.
A wafer substrate suitable for a device process preferably has few agglomerated defects other than point defects such as vacancy defects or self-interstitial defects in an active device region which is formed to several micro layers from a wafer surface.
Conventionally, as disclosed in KR Laid-open Patent Publication Nos. 2001-6182, 2001-6227 and 2001-6229, a vacancy concentration increases from a circumferential part towards a central part of a single crystal ingot, whereas an interstitial concentration decreases from the circumferential part towards the central part of the single crystal ingot, because a axial temperature gradient (G0) of the crystal has the function of G0=c+ax2. An insufficient out-diffusion in the vicinity of the circumferential part of the single crystal ingot results in crystal defects of interstitial characteristic, for example a large dislocation pit (LDP), whereby a crystal growth is generally performed in the state of a high vacancy concentration of the central part. As a result, crystal defects, for example voids or oxidation induced stacking faults (OiSFs), occur in a central part of a wafer where the vacancy concentration is much higher than an equilibrium concentration. However, reduction of a cooling rate of a crystal for a sufficient interstitial out-diffusion may require an additional hot zone. Further, it may reduce a growth velocity of a single crystal ingot, thereby decreasing productivity.
Another prior arts teach controlling the temperature distribution of a silicon single crystal ingot for producing a high quality silicon single crystal ingot. For example, JP Patent Application H2-119891 discloses controlling the temperature distribution of a central part and a circumferential part of a silicon single crystal ingot employing a hot zone of a high temperature region during cooling a single crystal, thereby reducing lattice defects of a silicon single crystal ingot caused by strain of solidification. This art intends to increase a solidification rate in the growth direction of a single crystal while reducing lattice defects by a cooling sleeve. JP Patent Application H7-158458 discloses controlling the temperature distribution in a crystal and a pulling rate of the crystal. JP Patent Application H7-66074 discloses controlling the defect density by improving a hot zone and controlling a cooling rate. JP Patent Application H4-17542 and KR Laid-open Patent Publication No. 2001-6229 disclose changing a hot zone and controlling the cooling rate to prevent formation of crystal defects by diffusion of point defects. KR Laid-open Patent Publication No. 2002-82132 discloses improving a heat shield and a water cooling tube to increase productivity of a high quality single crystal.
However, because these arts are based on a solid state reaction, the arts have following drawbacks. First, the arts have various limitations in achieving a high quality silicon single crystal. For example, KR Laid-open Patent Publication No. 2001-6229 discloses reducing the point defect density by sufficient diffusion reaction of supersaturated point defects at a high temperature region before the supersaturated point defects are grown to crystal defects. However, the art requires the temperature maintenance time of 16 hours or more, which is possible only in theory but not applicable in practice.
Second, the arts do not have substantial effects. As proposed in JP Patent Application H5-61924 and Eidenzon et al., titled “Defect-free Silicon Crystals Grown by the Czochralski Technique, Inorganic Materials”, Vol. 33, No. 3, 1997, pp. 272-279, an attempt was made to grow a 200 mm silicon single crystal ingot while periodically changing a pulling rate of a crystal, but a desired quality of a silicon single crystal ingot could not be obtained and further a single crystal growth process was instable.
Third, an invention based only on a solid state reaction theory cannot achieve a high productivity. For example, KR Laid-open Patent Publication No. 2001-101045 has designed optimum heat shield and water cooling tube, however, a pulling rate is in practical about 0.4 mm/min, thereby resulting in a low productivity.
Further, the above-mentioned arts have a low productivity of a high quality single crystal.